1. Field of the Invention
The present invention relates to an auto focus system, and in particular, to the auto focus system capable of focusing while following a desired target subject by automatically moving a target position (range) of an auto focus (AF) in a photographing range of a camera.
2. Description of the Related Art
As for an image pickup system such as a TV camera which converts an image of a subject to an electrical signal (image signal) with an image pickup device (CCD or the like), a contrast method is generally adopted as an auto focus (hereafter, referred to as AF) method. The contrast method is the method whereby a contrast of a subject image is detected from the image signal captured by an image pickup device and a focus of a photographing lens is controlled to obtain the highest contrast so as to automatically focus on the best focusing state (focused state).
In many cases, the AF such as the contrast method targets only the subject in a part of a photographing range of the camera rather than targeting the entirety of the subject in the photographing range. The AF of the contrast method extracts the image signals in the range targeted by the AF out of the image signals captured by the entire image pickup area of the image pickup device, and controls the focus to obtain the highest contrast of the image signals in the extracted range so as to limit the target of the AF only to the subject in a part of the range. This specification describes as an AF area the range of the subject targeted by the AF out of the photographing range of the camera or an image range of the subject targeted by the AF of the entire range of a photographing image (screen thereof) of the camera, and describes a frame showing an outline of the AF area as an AF frame.
In the case where a single AF area is fixedly set up at a predetermined position in the photographing range, a rectangular AF area is fixedly set up at the center of the photographing range as a standard. However, there is also the known case where the position of the AF area is changeable by an operation of an operator (refer to Japanese Patent Application Laid-Open No. 2002-365519).